Thermal desensitization of photosensitive systems

ABSTRACT

Exposed photosensitive elements are thermally desensitized to stabilize the image produced thereon.

Q Umted States Patent [151 3,661,583 Van Der Voorn 1 May 9, 1972 [54] THERMAL DESENSITIZATION OF [56] References Cited EN ITI PHOTOS S VE SYSTEMS UNITED STATES PATENTS [721 Invent: Rochester 2,220,882 1 1/1940 Bennes ..96/65 [73] Assignee: Eastman Kodak Company, Rochester, 3 9/ 19 C a .-..96/88 N Y. 2,835,577 5/1958 Levy ..96/107 [22] Filed: 1970 Primary Examiner-Norman G. Torchin [21 1 APPL 88 092 Assislam E.\'aminerWon H. Louie, Jr.

AttorneyW. H. J. Kline, B. D. Wiese and H. W. Myllus [52] U.S. CI ..96/48, 96/50, 96/88, [57] ABSTRACT Exposed photosensmve elements are thermally desensmzed to [5 1] Int- ..G03c Stabiliz e im ge p oduced thereo [58] Field of Search... ..96/88, 48, 48 PD, 107, 50

20 Claims, No Drawings THERMAL DESENSITIZATION OF PHOTOSENSITIVE SYSTEMS This invention relates to non-silver-halide photographic elements and to the stabilization of images produced on photosensitive elements comprising a reducible metal oxide or sulfide, such as lead monoxide.

it has been proposed to sensitize photographic elements comprising a reducible metal oxide or sulfide, such as yellow lead monoxide, and a binder to produce a visible image directly upon exposure to radiation. The sensitization of such photosensitive elements to print-out can be provided by incorporating a sensitizing agent in the dispersion prior to coating thereof onto the support. The metal oxide and sulfide sensitizing agents include hydroxy carboxylic acids, mercapto carboxylic acids, substituted benzoic acids, hydroxy-benzenes, polyhydric alcohols, hydroxyketones, thiols and the like.

When such sensitized photographic elements are subjected to exposure by actinic radiation, particularly in the ultraviolet range, they print-out with highly desirable speed and good image densities. However, one difficulty experienced with such photographic elements is the production of background density upon further exposure to room light with concomitant lowering of image discrimination. It has been suggested that such difficulties could be overcome by overcoating the exposed photographic element with an ultraviolet light-absorbing composition or stabilizing lacquer in order to prevent density build-up in the background. However, it would be highly desirable to eliminate the necessity and expense of such additional layers and techniques. Furthermore, it would be highly desirable to provide an improved method for fixing images obtained in such sensitized coatings.

Accordingly, it is an object of the present invention to provide an improved method for fixing images obtained on photosensitive metal oxide or sulfide elements.

Another object of the invention is to provide a method for the stabilization of print-out exposure coatings of lead monoxide to further action by visible or ultraviolet radiation without the employment of additional chemicals in the form of solutions or overcoats.

These and other objects of the invention are accomplished by providing a process for the stabilization of an imagewise exposed photosensitive element comprising a support, a metal.

oxide or sulfide, a binder, and a chemical sensitizing agent which has substantially no spectral absorption beyond that of said metal oxide or sulfide, and functions as a hydroxyl scavenging compound, which process comprises subjecting the photographic element to the sole action of heat.

Surprisingly, it has been found that an exposed photosensitive metal oxide or sulfide layer such as lead monoxide can be stabilized to the formation of background density by simply subjecting the exposed photographic element to elevated temperatures, e.g., 100-200 C. or higher. This process avoids the use of solutions and/or overcoats containing costly ultraviolet or blue light-absorbing compositions.

By the term photosensitive metallic oxides and sulfides" we mean to include the reducible light-sensitive oxides and sulfides of such metals as, for example, silver, lead zinc, mercury, titanium, tin, cadmium, bismuth, arsenic, germanium, tungsten, antimony, indium, and thallium. Of particular value in this invention is yellow lead monoxide, while other preferred light-sensitive metallic oxides and sulfides include bismuth trioxide, tungstic acid, and silver doped zinc-cadmium sulfide. Lead monoxide, i.e., yellow lead monoxide, PhD,

is a particularly effective member of this class of photosensi-- tive materials. Lead monoxide, for example, has an inherent sensitivity to radiation having a wave length of up to 450 millimicrons. This sensitivity may be extended further into the visible spectrum through the use of spectrally sensitizing dyes. Thermal desensitization of the lead monoxide photosensitive elements of the present invention may be conducted at any suitable temperature that will stabilize the image present on the exposed photographic element and prevent additional printout upon further exposure to radiation to which said element is sensitive at ambient temperatures. Suitable temperatures for the heat treatment may be, for example, in the range of from about 75 to about 350 C., preferably from about to about 300 C. Temperatures from about 180 to about 200 C. are especially preferred for the purposes of the present invention.

The time periods for the treatment for desensitizing the lead monoxide or other photosensitive element to, the formation of additional image density may be varied depending upon the temperatures employed. Thus, the heating period may be, for example, suitably up to about-5 minutes, and periods of time up to about 1 minute are preferred. Higher temperatures require shorter treatment periods and, generally, longer heating times at lower temperatures are required to obtain the same degree of desensitization. However, the higher temperatures and shorter heating periods are preferred, since when heating is carried out for longer periods, the net image discrimination decreases because the background begins to darken slightly.

Any suitable means of thermally desensitizing the lead monoxide photosensitive elements may be employed. Thus, for example, the photosensitive element may be placed in a heated enclosure, or treated with infrared lamps, heated rollers, steam, or the like.

' A suitable chemical sensitizing agent may be incorporated into the photosensitive coating, or an adjacent layer, in order to provide or improve the sensitivity of the coating to yield a visible image upon exposure to radiation. As previously mentioned, such sensitizing agents include alpha-hydroxy carboxylic acids, mercaptocarboxylic acids, substituted benzoic acids, e.g., hydroxybenzoic acids, aminobenzoic acids, mercaptobenzoic acids, methoxybenzoic acids, hydroxybenzenes, polyhydric alcohols, hydroxyketones, thiols and the like.

Suitable sensitizing agents include carboxylic acids and the esters of such acids, so selected as to have substantially no absorption of incident radiation beyond that of the photosensitive material employed, for example,

glycolic acid,

lactic acid,

methyl lactate,

Z-methyllactic acid,

methyl Z-methyl lactate,

DL mandelic acid,

pyruvic acid,

. oxalic acid,

3-hydroxybutyric acid,

citric acid,

2-phenoxypropionic acid,

pivalic acid,

Z-methylalanine,

malonic acid,

succinic acid,

o-hydroxy phenyl acetic acid,

2,5-dihydroxy phenyl acetic acid,

glutaric acid, and the like.

The carboxylic acids, o-hydroxyphenyl acetic acid, glycolic acid, lactic acid, 2-methyllactic acid, DL mandelic acid, pyruvic acid, and oxalic acid are preferred, with lactic acid, 2- methyllactic acid and oxalic acid being especially preferred as lead monoxide sensitizers.

A preferred class of sensitizers are the mercaptocarboxylic acids, for example,

Z-mercaptoisobutyric acid,

4-mercaptobutyric acid,

mercaptosuccinic acid, and the like.

0f the mercapto carboxylic acids, 4-mercaptobutyric acid and mercaptosuccinie acid are preferred for present purposes, with mercaptosuccinic acid being especially preferred.

Additional sensitizers of the present invention comprise substituted benzoic acids, including hydroxybenzoic acids, and their esters for example,

- salicylic acid,

2,4-dihydroxybenzoic acid, methyl salicylate,

2,5-dihydroxybenzoic acid,

3,4,5-trihydroxybenzoic acid,

m-hydroxybenzoic acid,

o-hydroxybenzoic acid,

2,4,6 -trihydroxybenzoic acid,

2,3,4-trihydroxybenzoic acid, and the like.

Likewise, amino benzoic acids, such as anthranilic acid; methoxybenzoic acids, such as o-methoxybenzoic acid; and mercaptobenzoic acids such as o-mercaptobenzoic acid are suitable for the purposes of the present invention.

The preferred benzoic acids include salicylic acid, 2,4- dihydroxybenzoic acid, methyl salicylate, anthranilic acid, mercaptobenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, and 3,4,5-trihydroxybenzoic acid. Especially preferred benzoic acids include salicylic acid, 2,4-dihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, and 2,4,6- trihydroxybenzoic acid.

Still further sensitizing agents include hydroxybenzenes, polyhydric alcohols, hydroxyketones, and thiols, such as phenol,

hydroquinone, I

resorcinol 1,3-benzenediol),

pyrogallol l,2,3benzenetriol),

phloroglucinol 1,3,5-benzenetriol),

o-aminophenol,

m-aminophenol,

3-hydroxy-2 butanone,

l ,3-dihydroxy-2-propanone,

4-hydroxy-2-butanone,

hexanethiol,

o-aminobenzenethiol,

l ,3-naphthalenediol,

Z-nitroresorcinol,

ethylene glycol,

glycerol,

2,2'-oxydiethanol,

3-mercapto-1 ,2-propanol, and

2,3-dimercaptopropanol.

The polyhydroxybenzenes are preferred and phloroglucinol is especially useful. Additional compounds which have a sensitizing effect on lead monoxide include m-phenylene diamine,

benzidine dihydrochloride, thiourea, and

thiosemicarbazide.

Other sensitizing agents include oxamide,

hydrazine dihydrochloride,

ascorbic acid,

2-mercaptobenzothiazole,

sodium formaldehydebisulfite,

sodium sulfite,

sodium dithionite,

S-hydroxyquinoline,

ethylene diamine tetraacetic acid,

dimethylglyoxime,

2,4-pentanedione, toluene-3,4-dithiol zinc salt, thiobenzanilide,

bis (dimethylthiocarbamyl)disulfide,

ethyldisulfide,

benzyldisulfide, and the like.

Certain sensitizing agents actually render the elements in which they are employed heatand infrared-sensitive in addition to being photosensitive. Thus, when yellow lead monoxide is dispersed in a polymeric binder in the presence of certain sulfur compounds, including mercaptoacetic acid, 3-mercaptopropionic acid, methylmercaptoacetate, thioglycolic acid B-aminonaphthalide, diethyldithiocarbamic acid sodium salt, sulfinoformamidine, sodium formaldehydesulfoxylate,

rendered heat-sensitive by such compounds are very useful in that they may be employed as photosensitiveelements, or as heat-sensitive elements which may be employed, for example, in hot needle recording. However, such heat-sensitive lead monoxide coatings are not susceptible to being thermally desensitized according to the present invention. Accordingly, such coatings are not encompassed by the present invention, and when the term sensitizing agents is employed herein, such sensitizing agents are intended to exclude those which are capable of yielding heat-sensitive elements, i.e., heatdevelopable elements.

The sensitizer may be provided in the system in any manner so long as it is present when the photosensitive element is subjected to exposure. The sensitizers may conveniently be incorporated into dispersions of the metal oxide or sulfide and binder prior to forming a coating of the photosensitive material on a support. Likewise, coated photosensitive elements may be'passed through a bath containing the sensitizing agent prior to exposure, or the sensitizing agent may be provide in an overcoat or undercoat layer which is adjacent the photosensitive layer. Preferably, the sensitizing agent is incorporated in the photosensitive metal oxide or sulfide layer.

Suitable amounts of the sensitizing agents include between about0.001 and, about 10 grams of sensitizer per gram of photosensitive compound, preferably between about 0.005 and about 1.5 gram per gram of metal oxide or sulfide.

As previously mentioned, the yellow lead monoxide is dispersed in a binder. Any suitable binder may be employed and suitable polymeric binders include hydrophilic as well as hydrophobic binders, such as copolymers of butadiene and styrene, polyvinyl butyral, polyvinylacetate, polyvinyl chloride latex, ethyl cellulose, gelatin and the like.

Lead monoxide photographic elements may be sensitized to provide upon exposure a visible image within a relatively short time, e.g., below about 1 minute, or as little as 30 seconds.

Furthennore, the exposure time may be materially shortened if the sensitized lead monoxide photographic element is exposed to form a latent or lowdensity image and thereafter physically developed. Thus, the sequential use of physical development permits the employment of short exposure times of much less than a minute, for example, from about 2 to about 15 seconds. Even at such short exposure times as 5 seconds or 2 seconds, good image density is obtainable.

The physical developer solution employed may be any developer previously employed in the physical development of aphotographic image. For example, a developer solution containing at least 5 X 10 moles per liter of a reducible metal salt such as a silver salt, may be employed which solution may also contain one or more silver halide developing agents known in the art. The reducible metal ions may be any of those which can be reduced to the metal by any of the wellknown photographic developing agents. In conventional physical developer solutions, the reducible metal salt is a water soluble silver salt, usually silver nitrate. However, other soluble silver salts may be used as well as salts of metals such as copper, mercury, platinum, gold, palladium, etc.

The photographic developing agent may be a polyhydroxybenzene developer or the like. Typical useful developers include hydroquinone, catechol, ascorbic acid, isoascorbic acid, pyrocatechol, gallic acid, gentisic acid, pyrogallol, etc. P-

phenylenediamine type silver halide developers may be used.

In addition, aminophenol types may be used such as pmethylaminophenol sulfate, o-aminophenol, .p-hydroxyphenolglycine, etc.

It will be appreciated that compounds known as auxiliary developers may be used also, such as the B-pyrazolidone type materials, and a combination may be used of more than one type of various types referred to above as well as a combination of developers with auxiliary developing agents.

Thus, the physical developer essentially contains an aqueous solution of a water soluble metal salt reducible to metal and a photographic developing agent for reducing the metal salt to metal. Suitable physical developers are described in [1.8. Pat. No. 3,390,998 to Cole which issued on July 2, 1968, and is hereby incorporated by reference.

The yellow lead monoxide or other suitable photosensitive metal oxides or sulfides may be provided in any suitable state of subdivision. Thus, lead monoxide, for example, may be provided in the form of particles having a particle size of between about 1.0 micron and l50 microns, preferably between about 2.5 and about 50 microns.

The sensitizing agents may be added to or incorporated in coatings with any of the well-known techniques commonly employed in the formation of dispersions and coatings therefrom. In the preparation of dispersions of the photosensitive material and a sensitizer, it may be desirable to include a small amount of a surface active agent which will act to improve the dispersion and may also improve the sensitivity.

Surfactants which are useful in the present invention include, for example, non-ionic surfactant species which are adsorbed at the boundary surface of two phases. Suitable amounts of the surfactant include between about 0.01 percent by weight and about 20 percent by weight. Suitable surfactants include alkylaryl polyether alcohols, sulfonates and sulfates (commercially available from Rohn & Haas as Triton X, for example), N-alkyl trimethylene diamines wherein the alkyl group is derived from coconut, soya or tallow fatty acids (commercially available from Armour & Co. as Duomeen S), dihexyl sodium sulfosuccinate (commercially available from American Cyanamid as Aerosol OT), polyethylene glycols and the like. Likewise, solvents such as toluene, methyl alcohol and the like are also suitable for incorporation and improvement of the dispersions.

Photographic elements to be stabilized according to the present invention may be formed by placing a coating of the photosensitive maten'al upon any available support material suitable for making photographic elements. Suitable support materials include paper, glass, metal, various polymeric materials such as cellulose esters, e.g., cellulose acetate, cellulose acetate butyrate, polycarbonates, polyesters, e.g., polyethylene terephthalate, polyethylene-coated supports, and the like.

The sensitized photographic elements to be stabilized have good speed and image discrimination when exposed to visible, ultraviolet or X-ray radiation. Through the use of suitable spectral sensitizers, such as spectrally sensitizing dyes, it is possible to extend the sensitivity of certain of the metallic 0xides or sulfides well beyond their inherent sensitivity, as is the case with lead monoxide. The term image discrimination" refers to the difference between reflection densities of image and background areas.

A loss of image discrimination and background print-out upon continued exposure of the photographic element to room light can be prevented by the simple heat treatment of the present invention.

The following examples further illustrate preferred embodiments of the present invention.

EXAMPLE 1 A dispersion is prepared which comprised 6 grams of fumed litharge (yellow lead monoxide from the Evans Lead Corporation), grams of a percent solution of a butadiene-styrene copolymer in toluene, 2 milliliters of a 0.06 gram per milliliter solution of Triton X-100 (a non-ionic surfactant) in toluene, 8 milliliters of toluene, and 0.6 gram of mercaptosuccinic acid (a lead monoxide sensitizing agent). Triton X-l00 is a water soluble alkyl aryl polyether alcohol in liquid, anhydrous form, available from Rohm and Haas. It has a viscosity of 240 centipoises at 25 C./25 C. and has a cloud point of l63-l78 F. for a 1 percent solution in distilled water.

The mixture is subjected to agitation for a period of 2 hours in a (SO-milliliter bottle containing 10 grams of one-eighth inch diameter ceramic balls. The resulting dispersion is coated onto a clay coated paper support at a wet thickness of 4 mils. Two samples of this coating are exposed for 1 minute to a Blak Ray ultraviolet lamp, Model UVL22, without filter glass (manufactured by Ultraviolet Products Inc.) at a sample-to-bulb distance of l centimeter.

The resultant reflection densities are measured with an Eastman Kodak Reflection Transmission Color Densitometer, Model l0-K. The net discrimination between image density and background density is 0.57.

One of the samples is heated at a temperature of 190 C. for a period of 1 minute. Next, both samples are exposed to fluorescent room lights for a period of 1 hour. The net image discrimination of the unheated sample drops to zero, while that of the heated sample is 0.49.

EXAMPLE 2 A lead monoxide coating is prepared according to the procedure of Example 1, with the exception that 0.5 gram of mercaptosuccinic acid is employed. The dispersion is coated onto a clay coated paper support and is exposed in the manner described in Example 1. The net image discrimination of each sample is measured as before and found to be 0.60. Exposed samples are heated at a temperature of 190 C. for varying lengths of time. After 6 A hours of exposure to fluorescent room lights, the net image discrimination for samples heated for three minutes or longer measures 0.48 or higher. On an unheated sample, the background density is 0.04 unit higher than the image density.

EXAMPLE 3 A mixture containing 6 grams of fumed litharge, one gram of sodium metaphosphate and 10 milliliters of water is agitated on a paint shaker with 10 grams of Va inch diameter ceramic balls for a period of 30 minutes. Next, 2 grams of a plasticized polyvinyl chloride latex emulsion containing approximately 53 percent solids is added and the mixture is stirred. To this dispersion, 0.6 gram of phloroglucinol is added and the dispersion is stirred and coated onto a paper support. The coating is exposed in the manner described in Example 1 and yields a net discrimination of 0.85.

A second sample which is produced in the same manner is identically exposed and is heated for 1 minute at 190 C. The resulting net image discrimination is 0.83. After a 2-hour exposure to fluorescent room light, the net image discrimination is 0.80, while that for the unheated sample is 0.20.

After 64 hours of exposure to fluorescent room light, the net image discrimination of the heated sample measures 0.36, while the image discrimination of the unheated sample measures 0.04 (i.e., the background density is 0.04 density units higher than that in the image area).

The following examples are presented for comparative purposes.

EXAMPLES 4-14 Various coatings are prepared from dispersions containing lead monoxide and certain sulfur compounds as additives. The dispersions are prepared by admixing 6 grams of fumed litharge, 10 grams of a 15 percent solution of a butadienestyrene copolymer in toluene, 7 milliliters of toluene, 2 mil liliters of a solution of 60 grams of Triton X-l00 per liter of toluene, and 0.6 gram of the sulfur compound. Ceramic balls having a inch diameter are added to the admixture and it is placed on a paint shaker for a period of 1 hour. The resulting dispersion is coated at a wet thickness of 4 mils on clay-coated paper. I

Samples of the coatings containing the sulfur sensitizing agents are placed in contact with an aluminum surface at a temperature of 190 C. for periods of 60 and seconds, respectively. The image reflection densities are measured as before. The results are set forth in Table 1, below:

TABLE I Contact Time Example Sensitizing Agent 60 sec. 120 sec.

4 Mercaptoacetic acid 0.25 0.82 5 B-mercaptopropionic acid 0.96 1.10 Y 6 Methylmercaptoacetate 0.65 0.80

7 Thioglycolic acid fl-aminonaphthalide 0.45 0.75 8 Diethyldithiocarbamic acid, 0.58

sodium salt 9 Sulfinoformamidine 0.47 10 Sodium formaldehydesulfoxylate 0.51 l l Thioacetanilide 1.00

The coatings of Examples 9-11 turn black much faster than the other coatings. Darkening occurs in the coatings of Examples 9 and 10 within 1 second and in Example 1 1 it occurs within 2 seconds. The change in reflection densities upon continued heating differs markedly, as shown in Table 2, below:

-TABLE 2 Contact Time Example Sensitizing Agent 5 sec. 10 sec. 20 sec.

12 Sulfinoformamidine 0.47 0.48 0.48 13 Sodium formaldehydesulfoxylate 0.51 0.52 0.53 14 Thioacetanilide 0.53 0.69 0.84

sensitive, and would accordingly be inappropriate for use with s the stabilization method of this invention, as illustrated by the following example.

1 EXAMPLE 15 Coatings of the sensitizing agents of Examples 4-14 are prepared in the manner described therein. However, the

coatings are exposed for 1 minute to a Blak Ray ultraviolet lamp Model UVL22 at a sample-to-bulb of l centimeter. The reflection densities are measured as before.

Most of the coatings give a visible print-out image after exposure of 1 minute. However, because the coatings are highly heat-sensitive, the net print-out density is reduced when the photographic elements are thereafter subjected to the heat treatment of the present invention. Accordingly, the thermal desensitization method of the present invention cannot be employed with such heat-sensitive coatings.

Similar results are obtained when other photosensitive metallic oxides or sulfides are substituted for the lead monoxide employed above, such as bismuth trioxide, tungstic acid, or silver doped zinc-cadmium sulfide. Thus, stabilized visible images may be prepared from photosensitive elements comprising a support, a binder, and a metal oxide such as bismuth trioxide, or tungstic acid, or a sulfide such as zinc-cadmium sulfide which has been doped with 100 ppm of silver.

' The invention has been described in detail with'particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

What is claimed is:

. 1. A process for the stabilization of an imagewise exposed photosensitiveelement bearing a visible image to the recording of background density, said photosensitive element comprising a support, a binder, a photosensitive reducible metal oxide or sulfide of a metal selected from the group consisting of silver, lead, zinc, mercury, titanium, tin, cadmium, bismuth, arsenic, germanium, tungsten, antimony, indium, and thallium and a sensitizing agent for said metal oxide or sulfide having substantially no spectral absorption beyond that of said metal oxide or sulfide, said sensitizing agent selected from the group consisting of carboxylic acids, carboxylic acid esters, hydroxy benzenes, polyhydric alcohols, hydroxyketones, alkane thiols and aryl thiols which process comprises subjecting said imagewise exposed element to a temperature of from about to about 350 C.

2. A process as set forth in claim 1 wherein said photosensitive metal oxide or sulfide is selected from the group consist ing of lead monoxide, bismuth trioxide, tungstic acid, and silver doped zinc-cadmium sulfide.

3. A process according to claim 2 wherein said sensitizing agent is a carboxylic acid or ester thereof.

4. A process as set forth in claim 3 wherein said carboxylic acid is selected from the group consisting of o-hydroxyphenyl acetic acid, glycolic acid, lactic acid, 2-methyllactic acid, DL mandelic acid, pyruvic acid, and oxalic acid.

5. A process as set forth in claim 4 wherein said metal oxide is lead monoxide.

6. A process as set forth in claim 2 wherein said sensitizing agent is a mercaptocarboxylic acid selected from the group consisting of 2-mercaptoisobutyric acid, 4-mercaptobutyric acid, and mercaptosuccinic acid.

7. A process as set forth in claim 6 wherein said metal oxide is lead monoxide.

8. A process as set forth' in claim 2 wherein said sensitizing agent is a benzoic acid or ester selected from the group con sisting of salicylic acid, 2,4-dihydroxybenzoic acid, methyl salicylate, anthranilic acid, o-mercaptobenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid and 3,4,5-trihydroxybenzoic acid. t

9. A process as set forth in claim 8 wherein said metal oxide is lead monoxide.

10. A process as set forth in claim 2 wherein said sensitizing agent is selected from the group consisting of hydroquinone, resorcinol, pyrogallol, phloroglucinol, ethylene glycol, and glycerol. v

11. A process as set forth in claim 10 wherein said metal oxide is lead monoxide.

12. A process as set forth in claim 2 wherein said element is subjected to a temperature of from about 180 to about 200 C.

13. A process as set forth in claim 12 wherein said metal oxide is lead monoxide.

14. A method for the production of a visible image which comprises imagewise exposing to actinic radiation a photosensitive element, comprising a support, a binder, and lead monoxide, in the presence of a chemical sensitizing agent having substantially no spectral absorption beyond that of lead monoxide and stabilizing said visible image by subjecting it to a temperature of from about 100 to about 350 C.

15. A method as set forth in claim 14 wherein said sensitizing agent is selected from the group consisting of lactic acid, 2-methyllactic acid, mercaptosuccinic acid, oxalic acid, phloroglucinol, and glycerol.

16. A process for the production of a visible image which comprises imagewise exposing a photographic element comprising a support and a yellow lead monoxide photosensitive layer, in the presence of a chemical sensitizing agent, to provide a latent image in said photosensitive layer, treating said photographic element with a physical developer to convert said latent image to a visible image, and stabilizing said photo graphic element by subjecting it to temperatures of from about 100 to about300 C.

17. A process as set forth in claim 16 wherein said temperatures are from about 180 to about 200 C.

18. A process as set forth in claim 16 wherein said sensitizing agent has substantially no spectral absorption beyond that of said yellow lead monoxide and does not render lead monoxide heatlsensitive.

19. A process as set forth in claim 18 wherein said sensitiz ing agent is selected from the group consisting of lactic acid, Z-methyllactic acid, mercaptosuccinic acid, oxalic acid, phloroglucinol, and glycerol.

20. A process as set forth in claim 19 wherein said sensitizing agent is phloroglucinol. 

2. A process as set forth in claim 1 wherein said photosensitive metal oxide or sulfide is selected from the group consisting of lead monoxide, bismuth trioxide, tungstic acid, and silver doped zinc-cadmium sulfide.
 3. A process according to claim 2 wherein said sensitizing agent is a carboxylic acid or ester thereof.
 4. A process as set forth in claim 3 wherein said carboxylic acid is selected from the group consisting of o-hydroxyphenyl acetic acid, glycolic acid, lactic acid, 2-methyllactic acid, DL mandelic acid, pyruvic acid, and oxalic acid.
 5. A process as set forth in claim 4 wherein said metal oxide is lead monoxide.
 6. A process as set forth in claim 2 wherein said sensitizing agent is a mercaptocarboxylic acid selected from the group consisting of 2-mercaptoisobutyric acid, 4-mercaptobutyric acid, and mercaptosuccinic acid.
 7. A process as set forth in claim 6 wherein said metal oxide is lead monoxide.
 8. A process as set forth in claim 2 wherein said sensitizing agent is a benzoic acid or ester selected from the group consisting of salicylic acid, 2,4-dihydroxybenzoic acid, methyl salicylate, anthranilic acid, o-mercaptobenzoic acid, m-hydroxybenzoic acid, o-hydroxybenzoic acid, 2,4,6-trihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid and 3,4,5-trihydroxybenzoic acid.
 9. A process as set forth in claim 8 wherein said metal oxide is lead monoxide.
 10. A process as set forth in claim 2 wherein said sensitizing agent is selected from the group consisting of hydroquinone, resorcinol, pyrogallol, phloroglucinol, ethylene glycol, and glycerol.
 11. A process as set forth in claim 10 wherein said metal oxide is lead monoxide.
 12. A process as set forth in claim 2 wherein said element is subjected to a temperature of from about 180* to about 200* C.
 13. A process as set forth in claim 12 wherein said metal oxide is lead monoxide.
 14. A method for the production of a visible image which comprises imagewise exposing to actinic radiation a photosensitive element, comprising a support, a binder, and lead monoxide, in the presence of a chemical sensitizing agent having substantially no spectral absorption beyond that of lead monoxide and stabilizing said visible image by subjecting it to a temperature of from about 100* to about 350* C.
 15. A method as set forth in claim 14 wherein said sensitizing agent is selected from the group consisting of lactic acid, 2-methyllactic acid, mercaptosuccinic acid, oxalic acid, phloroglucinol, and glycerol.
 16. A process for the production of a visible image which comprises imagewise exposing a photographic element comprising a support and a yellow lead monoxide photosensitive layer, in the presence of a chemical sensitizing agent, to provide a latent image in said photosensitive layer, treating said photographic element with a physical developer to convert said latent image to a visible image, and stabilizing said photographic element by subjecting it to temperatures of from about 100* to about300* C.
 17. A process as set forth in claim 16 wherein said temperatures are from about 180* to about 200* C.
 18. A process as set forth in claim 16 wherein said sensitizing agent has substantially no spectral absorption beyond that of said yellow lead monoxide and does not render lead monoxide heat-sensitive.
 19. A process as set forth in claim 18 wherein said sensitizing agent is sElected from the group consisting of lactic acid, 2-methyllactic acid, mercaptosuccinic acid, oxalic acid, phloroglucinol, and glycerol.
 20. A process as set forth in claim 19 wherein said sensitizing agent is phloroglucinol. 